The present invention relates to a multilayered air-fuel ratio sensing element preferably used for the air-fuel ratio control of internal combustion engines for automotive vehicles.
From the recent trend toward shortened sensor activation time and the positional restriction in installing the sensor (for example, installation to the exhaust gas pipe under a vehicle floor panel), improvement of the sensor warmup ability as well as downsizing of the sensor body are important goals to be attained.
Multilayered air-fuel ratio sensing elements, including united sensing and heating portions, have prospective properties to satisfy these requirements.
From the view point of electric insulation and heat transfer, conventionally proposed multilayered air-fuel ratio sensing elements generally comprise a heater-equipped alumina substrate and an oxygen ion conductive solid electrolytic body which are laminated integrally and sintered together. As having sufficient strength and excellent oxygen ionic conductivity, the partially stabilized zirconia is generally used as the oxygen ion conductive solid electrolytic body.
However, the multilayered air-fuel sensing elements have the following drawbacks because of their structural features including the different members (i.e., alumina and partially stabilized zirconia). When the sensing element is sintered in the manufacturing process or heated in the actual operating environment, a significant amount of thermal stress concentrates at the boundary between the alumina and the partially stabilized zirconia due to thermal expansion difference between them. This thermal stress triggers the cracks.
Enhancing the composition of the partially stabilized zirconia as well as increasing the strength and controlling the thickness of the alumina substrate will be effective to suppress the cracks from generating during the sintering step for manufacturing the sensing element from laminated green sheets of the alumina substrate and the solid electrolytic body (refer to the U.S. Pat. No. 5,447,618).
However, when the multilayered air-fuel ratio sensing element is installed in the internal combustion engine of an automotive vehicle, cracks may appear by the following mechanism.
The partially stabilized zirconic solid electrolytic body has a mixed phase structure including three different crystal structures referred to as a cubic (C) phase, a monoclinic (M) phase and a tetragonal (T) phase, with a small amount of additives. According to this phase structure, the T phase can transform into the M phase through the isothermal martensitic transformation (refer to T.fwdarw.M transformation).
The T.fwdarw.M transformation progresses rapidly when the partially stabilized zirconia is exposed to an atmosphere of approximately 200.degree. C. Presence of water (e.g., moisture or vapor) promotes the T.fwdarw.M transformation. Furthermore, the T.fwdarw.M transformation causes a volumetric change.
The operating environment of the air-fuel ratio sensing element incorporated in the automotive internal combustion engine can be regarded as repetitive heating and cooling cycles in a temperature range from the room temperature (20.degree. C.) to the exhaust gas temperature (1,000.degree. C.). The exhaust gas contains a large amount of vapor. Under such environment, the T.fwdarw.M transformation progresses smoothly.
When the T.fwdarw.M transformation occurs in the solid electrolytic body, cracks will appear along the boundary between the solid electrolytic body and the alumina substrate or along the surface of the solid electrolytic body.